FIG. 1 uses a circuit diagram to show the basic design of a generally known differential amplifier DV with a CMFB circuit CMFB. The differential amplifier DV has two inputs, to which a differential input voltage Vindiff is applied, and two outputs, at which a differential, amplified output voltage Voutdiff can be tapped off. The differential amplifier DV also has a control input into which a control signal Incntrl can be input. The CMFB circuit CMFB is in this case arranged in parallel between the outputs of the differential amplifier DV and the latter's control connection. The CMFB circuit CMFB is used for regulating a common-mode voltage produced at the output of the differential amplifier DV to a nominal value between the supply voltage. To this end, the CMFB circuit CMFB produces a control signal Incntrl which is derived from the output voltage Voutdiff and which is fed back to the control input of the differential amplifier DV. In differential amplifiers DV that are fully differential, the mean DC voltage ((VDD+VSS)/2) (common-mode voltage) produced at the output is not determined by the differential voltage Vindiff at the input, but rather is also determined by the controlled variable Incntrl.
Differential amplifiers may be in single-stage, two-stage or multistage form. For higher gains, two-stage or multistage differential amplifiers are therefore usually used.
FIG. 2 uses a circuit diagram to show a generally known differential amplifier with continuous-time common-mode control. This two-stage differential amplifier has an input stage A and an output stage B. To actuate the load for the input stage A, a continuous-time common-mode controller C is provided, to which an output signal tapped off from the output signal from the differential amplifier is supplied as controlled variable. Although a differential amplifier with such continuous-time CMFB control has a high level of stability in the signal which is to be amplified, this results (in order to provide very fast control in the common-mode controller C) in a very large current and hence in a high power consumption in the common-mode control amplifier C. For many applications, a high power consumption is not desirable and is frequently not acceptable, however. Fast control, on the other hand, is indispensable in order to prevent oscillations from arising in the control circuit.
U.S. Pat. No. 5,955,922 describes a differential amplifier with a CMFB circuit in which, unlike in the case of the continuous-time common-mode control just described, the common-mode value is set by switched capacitances (switched capacitor). Such differential amplifiers with switched capacitances have the particular advantage that a common-mode value is set by the switched capacitances, which means that controlling the differential amplifier thus consumes hardly any current. Setting the common-mode value using switched capacitances is therefore particularly advantageous, particularly for single-stage differential amplifiers.
Besides a single-stage differential amplifier, U.S. Pat. No. 5,955,922 also describes a two-stage or multistage differential amplifier with switched capacitances. However, the problem with such multistage differential amplifiers with switched capacitances is that they tend to have instabilities in the control as the number of amplifier stages increases. In particular, the common-mode value begins to oscillate as the number of differential amplifier stages increases. To rectify this, the differential amplifier described in U.S. Pat. No. 5,955,922 provides additional amplifier stages in the signal path. Although this makes the control more stable, it first of all has an adverse effect on the speed of the differential amplifier. Secondly, the differential amplifier consumes more current, which is something to be avoided as far as possible, particularly for a low power consumption.
DE 101 42 707 A1 describes a multistage differential amplifier circuit in which the CMFB circuit has switched capacitances. In this case, an input-side common-mode controller is provided. To ensure adequate stability for the common-mode control, the differential amplifier has a current source which, if too small a current flows in the feedback loop, feeds an additional current into the feedback loop. Although the differential amplifier circuit described therein thus has an adequately high level of stability, the current delivered by this current source is to a certain extent a control stop for the common-mode control. The problem in this case is that if the common-mode value has relatively large discrepancies—which is very often the case—the control in this circuit very quickly hits a control stop, which means that the control does not work or no longer works satisfactorily, however. The CMFB circuit described in DE 101 42 701 A1 therefore has the drawback of a very limited input range for the common-mode value which can be used.